1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
2. Description of the Related Art
As a small-area, thin semiconductor package (hereinafter simply called “a package”) that permits high-density packaging, there is available a package in which the flip-chip mounting method is adopted for an electrical connection between a semiconductor chip (hereinafter simply called “a chip”) and a wiring substrate (hereinafter simply called “a board”). In recent years, demand for this type of package has been increasing.
FIGS. 25 and 26 are diagrams showing a general semiconductor device in which the flip-chip mounting method is adopted. FIG. 25 is a sectional view and FIG. 26 is a plan view.
As shown in FIG. 25, the flip-chip mounting method is such that an electrode pad (not shown) formed on a surface (circuit surface) of a chip 101 and an electrode pad (not shown) formed on an interconnect (not shown) of a board 102 are bonded together via bumps 103 formed from solder, gold, copper and the like, whereby the chip 101 and the board 102 are connected together.
Because the bumps 103 have a certain height (on the order of 10 μm to 150 μm), usually a gap is generated between the mounted chip 101 and the board 102. An underfill resin 104 is generally filled in this gap. The filling of the underfill resin 104 is performed to protect a bond part bonded via the bumps 103 and a surface of the chip 101 and to prevent the bumps 103 from melting due to high-temperature treatment performed during the mounting of the package and shorting between the bumps 103.
The process of filling the underfill resin 104 into a gap between the chip 101 and the board 102 is often performed after the connection of the chip 101 to the board 102. This process is generally performed by causing the underfill resin 104 to fall in drops from near the chip 101 by use of a needle or the like in such a manner that the underfill resin 104 is caused to penetrate (injected) to between the chip 101 and the board 102 by the capillary phenomenon.
In almost all cases, the filling of the underfill resin 104 is performed either by injection from one point near the chip 101 or by injection while the needle is being moved along any one selected from the four sides of the chip 101 (the four sides forming a planar shaped outer circumference of the chip 101).
A portion composed of the chip 101, the board 102 and the bumps 103 is called a flip-chip mounted body. In the flip-chip mounted body, a bow has already occurred in the flip-chip mounting step, which is the step just before the step of injecting the underfill resin 104. If the board 102 is positioned below and the chip 101 is positioned above, this bow is an upward convex bow which is laterally symmetrical with reference to the center position 107 of the chip 101. For example, when the bumps 103 are formed from a lead-free solder, this bow occurs due to a difference in the coefficient of thermal expansion between the board 102 and the chip 101, which is generated during cooling to room temperature after the connection of the chip 101 to the board 102 by melting the bumps 103 at temperatures of not less than 250° C. The coefficient of thermal expansion of the board 102 is five to ten times the coefficient of thermal expansion of the chip 101. Incidentally, when the bumps 103 are formed from a lead-free solder, the effect of the underfill resin 104 on the bow is a limited one and, therefore, it may be thought that the greater part of the bow of the flip-chip mounted body is inherited from a bow which occurred in the flip-chip mounting step, which is a preceding step.
When a bow is large for such reasons as the thinness of the board 102, a flip-chip mounted body may sometimes be used as a package by the resin encapsulation of the flip-chip mounted body after the setting of the underfill resin 104 in order to correct the bow in the flip-chip mounted body. That is, in order to correct a convex bow existing after the setting of the underfill resin 104, for example, as shown in FIGS. 25 and 26, the extent of a bow on the whole package is lessened by the encapsulation with a resin for encapsulation 106 having a large contraction stress in such a manner as to cover an upper part of the flip-chip mounted body.
In a general package, the structure of the package is such that, as shown in FIGS. 25 and 26, the center position 108 of the board 102 (which is also the center of the package and the center of the flip-chip mounted body), the center position 107 of the chip 101, and the center position 109 of the resin for encapsulation 106 coincide with each other.
Incidentally, FIG. 26 shows the stage before the singulation of each package. The board 102 and the resin for encapsulation 106 are cut along dicing lines 115 shown in FIG. 26, whereby each package is separated and it is possible to obtain the semiconductor device shown in FIG. 25.
As with the semiconductor device shown in FIGS. 25 and 26, a semiconductor device in which the flip-chip mounting method is adopted is disclosed in Japanese Patent Laid-Open No. 2008-166373, for example.
Japanese Patent Laid-Open No. 11-017070 discloses a structure which is such that a chip is mounted on a first board by the flip-chip mounting method and is filled with an underfill resin and on this chip there are disposed a resin and a second board symmetrically with reference to the center of the chip, whereby thermal stress, particularly, the setting and shrinkage of the resins are kept in balance above and below the chip and a bow in the whole structure is compensated for.
A protruding portion of the underfill resin 104 which is formed in the circumference of the chip 101 is called a fillet 105. The shape of the fillet 105 depends on a method of injecting the underfill resin 104. In general, the fillet 105 is relatively large in a portion where the injection of the underfill resin 104 is performed in the circumference of one chip 101 and is relatively small in other portions. For this reason, generally, the position of the whole (filler outer circumference) of the underfill resin 104 is asymmetrical with reference to the center position 107 of the chip 101.
Specific examples of the shape of the fillet 105 are described with the aid of FIGS. 27A and 27B and FIGS. 28A and 28B. FIGS. 27A and 27B show the shape of a fillet 105 obtained when the underfill resin 104 was injected from one point, and FIGS. 28A and 28B show the shape of a fillet 105 obtained when the injection of the underfill resin 104 was performed while a needle was being moved along one side of the chip 101. FIGS. 27A and 28A are sectional views and FIGS. 27B and 28B are plan views.
As shown in FIGS. 27A and 27B, when the underfill resin 104 is injected, with the position of the needle for injecting the underfill resin 104 fixed at one point (the needle position 111 of FIG. 27B, a portion of the fillet 105 whose center is the needle position 111 becomes a large-sized portion 105a whose size is relatively large. In the fillet 105, a portion other than the needle position 111 and the surrounding portion thereof becomes a small-sized portion 105b whose size is smaller than the large-sized portion 105a. As shown in FIGS. 28A and 28B, when the underfill resin 104 is injected while the needle position 111 is being moved along one side 112 of the chip 101, a portion of the fillet 105 along the side 112 becomes a large-sized portion 105a and the other portion becomes a small-sized portion 105b. 
When the fillet 105 of the underfill resin 104 is formed asymmetrical with reference to the center position 107 of the chip 101 in this manner, also the distribution of the amount of the resin for encapsulation 106 in a package becomes asymmetrical according to the shape of the fillet 105. As a result of this, also the bow correcting effect of the resin for encapsulation 106 becomes asymmetrical. That is, in a portion where the fillet 105 is large (the large-sized portion 105a), a sufficient bow correcting effect cannot be obtained because the amount of the resin for encapsulation 106 becomes relatively small. Contrastingly, in a portion where the fillet 105 is small (the small-sized portion 105b), the bow correcting effect may sometimes become excessive because the amount of the resin for encapsulation 106 becomes relatively large.
In a general package, the center position 108 of the board 102, the center position 107 of the chip 101, and the center position 109 of the resin for encapsulation coincide with each other. In this case, when the fillet 105 of the underfill resin 104 is formed asymmetrical with reference to the center position 107 of the chip 101, the balance of the bow correcting effect after resin encapsulation becomes lost when the package is viewed as a whole. For this reason, as a result of this, for example, as shown in FIG. 29A, a semiconductor device (a package) may sometimes have an asymmetrical bow shape. Incidentally, FIG. 29A is a diagram showing, in an exaggerated manner, the bow shape of a package which obtained room temperature after resin encapsulation.
The contractive force of the resin for encapsulation 106 has a predominant effect on a bow in a flip-chip mounted body. For this reason, the bow shape of a package becomes asymmetrical mainly due to the extent of the bow correcting effect of the resin for encapsulation 106 in each portion of the package.
However, when the underfill resin 104 is a thermosetting resin, the setting of this resin proceeds at temperatures higher than room temperature (for example, on the order of 80° C. to 200° C.). For this reason, when the temperature of the underfill resin 104 returns to room temperature after setting, a thermal stress is generated between the underfill resin 104 and the board 102 due to differences in the coefficient of thermal expansion and the modulus of elasticity and as a result of this, the condition of a bow of a flip-chip mounted body which has originally warped due to a thermal stress during flip-chip mounting changes a little. This change is also affected, though slightly, by the shape of the fillet 105 formed in the circumference of the chip 101. If the board 102 is positioned below and the chip 101 is positioned above, before the setting of the underfill resin 104, the bow is symmetrical with reference to the center of the chip 101 and becomes convex upward. However, in a case where the center position 107 of the chip 101 and the center position 108 of the board 102 coincide with each other, when the underfill resin 104 sets, with the shape of the fillet 105, particularly, the size of the fillet 105 differing according to a portion in the circumference of the chip 101, a bow of the board 102 after setting becomes asymmetrical in the strict sense with reference to the center of the flip-chip mounted body (the center position 108 of the board 102). That is, although the effect of the shape of the fillet 105 of the underfill resin 104 on the bow of a flip-chip mounted body is a limited one, strictly speaking, also due to a thermal stress generated by the underfill resin 104, the bow of the flip-chip mounted body, hence the bow of the whole semiconductor device becomes asymmetrical. Furthermore, because the bow of the flip-chip mounted body, hence the bow of the whole semiconductor device becomes asymmetrical like this, the amount of bow of the flip-chip mounted body, hence the amount of bow on one side of the semiconductor device also increases.
A description will be given here of specific examples of values of physical properties for each of the resin for encapsulation 106, the underfill resin 104, the board 102, and the chip 101. For the resin for encapsulation 106, for example, Tg (the glass transition temperature) is 120 to 130° C., α1 (the coefficient of linear expansion at less than Tg) is 30 to 40 ppm/° C., and α2 (the coefficient of linear expansion at not less than Tg) is 80 to 120 ppm/° C.). For the underfill resin 104, for example, Tg is 135 to 145° C., α1 is 20 to 30 ppm/° C., and α2 is 80 to 100 ppm/° C. For the board 102, for example, Tg is 160 to 190° C., α1 is 15 to 30 ppm/° C., and α2 is 10 to 35 ppm/° C. For the chip 101, for example, α (the coefficient of linear expansion) is 3 to 5 ppm/° C. In general, when the coefficient of thermal expansion (concretely, for example, the coefficient of linear expansion) of the resin for encapsulation 106 is larger than the coefficient of thermal expansion of the underfill resin 104, the package bow reducing effect (the bow correcting effect) is dependent on the resin for encapsulation 106 (the contractive force of the resin for encapsulation 106 becomes predominant over the bow shape of the package). Also when Tg of the resin for encapsulation 106 is lower than Tg of the underfill resin 104, the package bow reducing effect (the bow correcting effect) is dependent on the resin for encapsulation 106 (the contractive force of the resin for encapsulation-106 becomes predominant over the bow shape of the package). That is, the lower Tg of the resin for encapsulation 106, the wider the temperature region (temperature range) in which the coefficient of thermal expansion (concretely, for example, the coefficient of linear expansion) of the resin for encapsulation 106 becomes larger than the coefficient of thermal expansion of the underfill resin 104, and in this sense, the lower Tg of the resin for encapsulation 106, the more the contractive force of the resin for encapsulation 106 will be predominant over the bow shape of the package.
Also from these values of physical properties, it is apparent that as shown in FIGS. 25 and 26, when the center positions 107, 108 and 109 of the chip 101, the board 102 and the resin for encapsulation 106, respectively, coincide with each other, a bow at room temperature does not become symmetrical (FIG. 29A), and that besides a bow of a package can exhibit an asymmetrical behavior also to a temperature response (a response of a bow to a temperature change).
When a semiconductor device is mounted on a wiring substrate and the temperature response of a bow at the temperature at which reflow is performed is asymmetrical, this can exert an adverse effect on the mountability of the semiconductor device. That is, when a semiconductor device is mounted on a wiring substrate (a motherboard, not shown), which is an end product, by melting BGA balls 110 (FIG. 25) of the semiconductor device (reflow), the package obtains temperatures (for example, 230° C. to 260° C.) higher than the setting temperatures of the underfill resin 104 and the resin for encapsulation 106. Therefore, the thermal stress balance in the interior of the package becomes lost and the bow of the package exhibits an asymmetrical behavior.
FIG. 29B shows, in an exaggerated manner, the bow behavior of a package during the reflow of mounting (solder melting temperature: for example, 230° C. to 260° C., as described above). Incidentally, as shown in FIGS. 29A and 29B, the direction of the bows of a package is inverted at room temperature (FIG. 29A) and solder melting temperatures (FIG. 29B). This is due to a difference in the coefficient of linear expansion between the board 102 and the chip 101. The bow of the package becomes almost zero at the temperature at which the chip 101 is flip-chip mounted on the board 102. However, because the board 102 has a larger coefficient of linear expansion than the chip 101, the amount of contraction of the board 102 becomes larger than that of the chip 101 at temperatures of not more than the flip-chip mounting temperature. Therefore, an upward convex bow occurs in the whole semiconductor device. Contrastingly, at temperatures higher than the flip-chip mounting temperature, i.e., during the reflow of mounting at temperatures higher than solder melting points, the amount of expansion of the board 102 becomes larger than that of the chip 101 and, therefore, a downward convex bow occurs in the whole semiconductor device.
When a package has an asymmetrical bow shape at room temperature, this has an effect on the transferability of a flux during mounting. Also, when an asymmetrical bow occurs during the reflow in mounting on a wiring substrate, this has an effect on the bondability of the BGA balls 110 to the wiring substrate or the connection reliability after mounting. As described above, asymmetry of the bow characteristics of a package results in a decrease in the margin of mountability, a decrease in mounting yield, and a decrease in mounting reliability. Such adverse effects of asymmetry of bows have become manifest particularly in packages in which recent thin boards of less than 400 μm are used.
As described above, it has been difficult to suppress the extent of bows of semiconductor devices when the center of an underfill resin deviates from the center of a chip due to the asymmetry of the fillet width of the underfill resin.